In microlithography, as is known, semiconductor structures are realized by means of the imaging of a structure, produced beforehand on a lithography mask, on a photoresist-coated wafer by means of the exposure and subsequent development of the photoresist.
After the fabrication of the lithography mask, the latter has various properties which change from mask to mask. During the imaging thereof, these various properties have a considerable influence on the photolithography. In this case, a distinction is made between general (global) and local properties, which are normally specified.
Examples of global properties are the deviation of the line width (CD target value) from the target dimension (offset error), and also the fluctuation in the deviation over the image field (uniformity error). In the case of alternating phase masks, there are structures which have phase-shifting properties in the case of coherent illumination. These phase shifters are realized by etching structures into the glass. Deviations from the ideal etching depth result in undesirable brightness deviations, which likewise represent a global mask property.
Local properties are defects on the mask which have either arisen as a result of faults during mask production, or else subsequent contaminants, e.g. due to dust.
Therefore, the aim in the production of lithography masks is to produce a lithography mask which fulfils all the requirements made of accuracy and freedom from faults. With reference to the local properties, the masks are at the present time tested by means of process control and quality measurements.
In the prior art, systematic fabrication faults of lithography masks, such as regular deviations of the line spacing (offset target area) and changes in the line width (uniformity error), are measured directly at the structures by means of various SEM measurements (scanning electron microscopy) on any points of the mask. This may be a time-consuming and unreliable evaluation process since the measurement errors and the poor repeatability of an SEM are disadvantageous. Thus, by way of example, the direction of an electron beam significantly influences the measurement result. For this reason, this method is not suitable for lithography masks etched into quartz since, in the case of these masks, there are no well-defined edges for the SEM line width measurement. A further problem is the contamination of the mask material by carbon deposits which arise during the measurement process.
For new mask technologies, such as interference masks, the mask evaluation becomes very difficult. Interference masks contain trenches which have been etched into the mask and effect a defraction (phase shift) of the transmitted light. However, there is no established method for determining the defraction and transfer parameters.
The etching depth of the trenches in alternating phase masks is controlled by employing a combination of depth measurement and optical measurement using an MSM microscope (microlithography simulation microscope). The measured values obtained with such a microscope are finally compared with line width measurements of patterned wafers in order to obtain an item of information about the value of the newly produced lithography masks.
A further aim is now to go over to defect inspection. Since some defects on the lithography mask are transferred to the chips shaped on the wafer, it is very important to ensure a fault-free mask structure. Since each process step in mask fabrication unavoidably generates some defects in the lithography mask, it is necessary to be able to inspect and repair lithography masks.
During mask inspection, the lithography mask is examined for defects and the defects found are classified according to their importance for the lithography and the influence on the functionality of the chip. Mask defects which jeopardize the functionality and reliability of the chip function must necessarily be repaired.
The test devices used at the present time utilize laser scanning microscopy in order to check lithography masks for defects. In this case, the mask surface is scanned with a laser beam and the reflected and transmitted light is measured. In the case of defect-free regions, the sum of the reflected and the transmitted light lies above a predefined value, whereas the light is scattered in the case of defects, so that the sum of the intensities differs from a predefined value.
In the case of high-end masks, this approximation is expanded in various ways; thus, by way of example, instead of the comparison with a reference signal (die-to-database), an adjacent chip is utilized as a reference for “die-to-die” inspection, the chip surface in each case being subdivided into arrays to be examined and the arrays then being compared in pairs.
In the case of standard-quality masks, this approximation is simple and efficient with regard to the inspection time and costs. In the case of high-end masks, and in particular in the case of interference masks, this approximation suffers from the fact that these utilize a different lower wavelength than during the concluding lithography step. However, since a lower wavelength means a lower resolution, the sensitivity with regard to defects is lower than during the concluding lithography process.
Another problem of the laser scanning method is that the interference masks (defraction gratings) are seen very differently during laser scanning and during optical imaging.
One solution to this problem consists in simulating the lithographic imaging process with a modified microscope. Such a type of lithography microscope is designated in an abbreviated manner by the product name MSM (microlithography simulation microscope) and is offered for example by the company ZEISS. The MSM has the advantage that it uses the same wavelength as during the lithography process, the same illumination of the lithography mask and the same optical specifications for the aperture angle of the projection optic.
In contrast to lithographic exposure, during which the image of the lithography mask is demagnified, the MSM magnifies the image of the lithography mask on to a sensor. Furthermore, the MSM can only image a small section of the mask simultaneously.
During the progressive scanning of the lithography mask, it is possible to record images thereof which essentially correspond to the intensity with which a wafer would be exposed.
If a fault has a major influence on the exposure of the resist, this would be evident from the aerial image which has been recorded by the MSM microscope. In order, however, to be able to utilize the microscope for the inspection, a fault signal which signals the defect is required. Consequently, the lithography mask must have various identical chip layouts, so that the microscope can compare these images in pairs or the entire image of a fault-free lithography mask is present for the comparison in a memory.
A disadvantage of this method is that each image of the lithography mask is influenced by the general noise, and that the ultimate fault image has a relatively high noise.
A further disadvantage of this method is the complexity thereof in that the entire image of a lithography mask has to be scanned into the memory of the computer, partially adjusted prior to a comparison and subsequently compared. This requires both a considerable storage capacity and considerable computation time.